I have a 1995 Dodge W3500 5.9 and about a year ago I noticed that at 70 MHP or above that the brake pedal becomes very hard to push and feels like that I have manual brakes. I have replaced the vacuum pump and have tested and it has the correct vacuum. I have replaced the power brake boost. If the speed is below the 70 mark the brakes work fine. Please if anyone has experienced this problem and found the fix respond.

 

That is a pretty wierd one. you might want to post the question in the 2nd gen forum with a 95 and all.

Are you getting much pulsation from the pedal? warped or thickness variable rotors might have a little to do with it.

might want to check the pad material too. some glaze or oil on the rotors might have something to do with it.

Maybe the ABS controller is getting some wierd signals at high speed and its messing with your brakes too.

Last thing i could shake a stick at might be wheel bearings, if they are loose they might have some affect changing at high speed.

 

Not sure, but if you have oversize tires and don't have the speedo adjusted, your ABS may be sending a bogus signal. Until I adjusted for my tire size, my brakes worked OK, but were a little squirrelly sometimes.

Why don't you fill in your signature so we'll know a little more about your truck.

EDIT: If you have ABS

 

I don' know, but it sounds almost rpm related.

Are you sure that the booster system is operating at the same rpm you are turning at 70mph??

I would check it with gauges at speed to be sure.

Dave